Plastics are the most widely used materials in the United States, surpassing steel, copper, and aluminum combined by volume. Among the various polymer processing methods, injection molding accounts for one-third of all materials processed. While plastics have become ubiquitous and have greatly improved our quality of life, the rising cost of fossil-based plastics, as well as the environmental burden, cannot be overlooked. Special injection molding processes that reduce material usage and energy consumption are highly desirable to cut production costs, save natural resources, and reduce permanent waste.
Microcellular injection molding is an emerging special injection molding process capable of producing foamed parts with many advantages. During the microcellular injection molding process, a supercritical fluid is introduced into a molten polymer prior to the polymer being injected into a mold. The polymer solidifies in the mold to form a desired component. The introduction of the supercritical fluid prior to injection of the polymer into the mold causes tiny bubbles to be distributed throughout the molded component. By providing tiny bubbles in the molded component, the amount of material necessary to mold the component is reduced, while the dimensional stability of the molded component is improved.
While the microcellular injection molding process saves on material cost and improves production efficiency as compared to conventional solid injection molding, the process does have certain limitations. By way of example, microcellular injection molding requires specially designed supercritical fluid delivery and dosing systems to be installed on the injection molding machine for the delivery of the supercritical fluid as a physical blowing agent. In addition, modifications need to be made to the injection molding machine itself, including the installation of a supercritical fluid delivery device and a special injection screw with mixing elements for effectively mixing the supercritical fluid with the liquid polymer. These two factors lead to an increase in capital investment, especially when a large number of injection molding machines need to be modified.
Alternatively, the microcellular injection molding process may performed be using gas-laden pellets processed by a conventional injection molding machines. More specifically, pellets of a polymer are saturated with a physical blowing agent in a high pressure vessel. These gas-laden pellets are introduced into the injection molding machine, which, in turn, are used to fabricate foamed injection molded parts. This type of process offers all of the aforementioned benefits of microcellular injection molding, yet requires a much lower investment in equipment.
While the microcellular injection molding process saves on material cost and improves production efficiency as compared to conventional solid injection molding, the process does have certain disadvantages and limitations. For example, components fabricated using the microcellular injection molding processes heretofore described exhibit relatively poor ductility and toughness. As a result, these components tend to be brittle, especially for thin-wall products. In order to improve ductility and toughness, modifiers such as fibers or rubber particles are commonly introduced during the foaming process of a component. However, while these modifiers do increase the ductility and toughness of the component, the modifiers are expensive thereby reducing the cost benefits of the microcellular injection molding process. Further, use of modifiers has been found to reduce the high temperature performance and increase the overall weight of the component.
Therefore, it is a primary object and feature of the present invention to provide a method for fabricating foamed, injection molded components with improved ductility over prior injection molded components.
It is a further object and feature of the present invention to provide a method for fabricating foamed, injection molded components which is compatible with prior microcellular injection molding processes.
It is a still further object and feature of the present invention to provide a method for fabricating foamed, injection molded components which is simple and which may be performed with standard injection molding machinery.
In accordance with the present invention, a method is provided of fabricating an injection-molded component. The method includes the step of introducing pellets and a first supercritical fluid into an injection barrel of an injection molding machine. The pellets include a first polymeric material and a second polymeric material. The pellets are plasticized within the injection barrel to form an injection material. The first polymeric material defines a first phase of the injection material and the second polymeric material defines a second phase of the injection material. The first phase and the second phase are immiscible. The injection material is injected into a mold to fabricate the injection-molded component.
Microcellular voids having diameters are formed in the injected-molded component. The injected-molded component may be subjected to tensile loading and secondary phase cavities having diameters may be formed in the injected-molded component in response to the tensile loading of the injected-molded component. The diameters of the microcellular voids are greater than the diameters of the secondary phase cavities. The microcellular voids have diameters greater than 10 micrometers and the secondary phase cavities have diameters less than 1 micrometer. Preferably, the microcellular voids have diameters in the range of 50 to 100 micrometers.
In a first arrangement, the first and second polymeric materials and the first supercritical fluid are heated to produce a melt. The melt is extruded and the pellets are formed from the extruded melt. Alternatively, the first supercritical fluid may be introduced into the injection barrel downstream of the pellets. By way of example, it is contemplated for the first polymeric material to be polypropylene (PP) and the second polymeric material to be high-density polyethylene (HDPE). The injection material includes at least 50% of the first polymeric material and less than 50% of the second polymeric material, and preferably, the first polymeric material is in the range of 60% to 85% of the injection material.
In accordance with a further aspect of the present invention, a method is provided for fabricating a foamed, injection-molded component. The method includes the step of plasticizing pellets including a first polymeric material and a second polymeric material within an injection barrel to form an injection material. The first polymeric material defines a first phase of the injection material and the second polymeric material defines a second phase of the injection material. The injection material is injected into a mold to fabricate the foamed, injection-molded component. The first phase and the second phase of the injection material are immiscible.
Microcellular voids having diameters are formed in the injected-molded component. The injected-molded component may be subjected to tensile loading and secondary phase cavities having diameters may be formed in the injected-molded component in response to the tensile loading of the injected-molded component. The diameters of the microcellular voids are greater than the diameters of the secondary phase cavities. The microcellular voids have diameters greater than 10 micrometers and the secondary phase cavities have diameters less than 1 micrometer. Preferably, the microcellular voids have diameters in the range of 50 to 100 micrometers.
In a first arrangement, the first and second polymeric materials and the first supercritical fluid are heated to produce a melt. The melt is extruded and the pellets are formed from the extruded melt. Alternatively, the first supercritical fluid may be introduced into the injection barrel downstream of the pellets. By way of example, it is contemplated for the first polymeric material to be polypropylene (PP) and the second polymeric material to be high-density polyethylene (HDPE). The injection material includes at least 50% of the first polymeric material and less than 50% of the second polymeric material, and preferably, the first polymeric material is in the range of 60% to 85% of the injection material.
In accordance with a still further aspect of the present invention, a method is provided for fabricating a foamed, injection-molded component. The pellets include a first polymeric material and a second polymeric material are plasticized within an injection barrel to form an injection material. The injection material is injected into a mold to fabricate the foamed injection-molded component. Microcellular voids having diameters are formed in the injected-molded component. The injected-molded component may be subjected to tensile loading and secondary phase cavities having diameters may be formed in the injected-molded component in response to the tensile loading of the injected-molded component. The diameters of the microcellular voids are greater than the diameters of the secondary phase cavities.
It is contemplated for the microcellular voids to have diameters greater than 10 micrometers and for the secondary phase cavities to have diameters less than 1 micrometer. Preferably, the microcellular voids have diameters in the range of 50 to 100 micrometers. The first polymeric material defines a first phase of the injection material and the second polymeric material defines a second phase of the injection material. The first and second phases of the injection material are immiscible. By way of example, it is contemplated for the first polymeric material to be polypropylene (PP) and the second polymeric material to be high-density polyethylene (HDPE). The injection material includes at least 50% of the first polymeric material and less than 50% of the second polymeric material, and preferably, the first polymeric material is in the range of 60% to 85% of the injection material.